1. Field of the Invention
The present invention generally relates to an image encoding apparatus and method, for compressing an input image according to the JPEG-2000 Standard, for example.
This application claims the priority of the Japanese Patent Application No. 2003-127409 filed on May 2, 2003, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
One of the typical techniques of image compression is defined in the JPEG (Joint Photographic Experts Group) standardized by the ISO (International Standards Organization). The JPEG uses the DCT (discrete cosine transform) to provide quality encoded images and decoded images in case a relatively high bit rate is assigned for the encoding. However, if the bit rate for encoding is smaller than necessary, the block distortion peculiar to the DCT will be remarkable and a resultant image will appear noticeably degraded in quality.
On the other hand, many researches have recently been done of the techniques of dividing an image into a plurality of frequency bands by a so-called filter bank including a high-pass filter and low-pass filter in combination and encoding each of the frequency bands. Of such techniques, the wavelet-transform coding is considered as a new promising technique which will take the place of the DCT (discrete cosine transform) because even a high compression results in no considerable block distortion as in the DCT.
For example, the JPEG-2000 Standard established as an international standard in January, 2001 adopts a combination of the wavelet transform and a high-efficiency entropy coding (bit modeling and arithmetic coding, both in bit planes), and has attained a greater improvement in efficiency of coding than the conventional JPEG (cf. Japanese Published Unexamined Patent Application No. 2002-165098).
These international standards defines only the design requirement for the decoder, not any for the encoder. Therefore, the encoder can freely be designed. On the other hand, since there exists no standard for any effective rate-control means to attain a target compression ratio, so it is most important to establish a technical know-how for attaining a target compression ratio.
In encoding an image whose resolution is 2000×1000 for example in the encoder defined in the JPEG-2000, however, it is common to control the bit rate for a minimum distortion of the entire image. On the other hand, in similar coding of n image whose resolution of 1000×500, which is a square of the above-mentioned resolution, the bit rate is also controlled for a minimum distortion of the entire image. This is a rate control based on the RD (rate-distortion) characteristic. Such rate control is versatile but disadvantageous in that the load of calculation is very great. Also, the technique of controlling a bit rate for a minimum numerical distortion is disadvantageous in that it cannot assure a highest quality of an image when viewed by the human being.
Further, for compressing an image of 2000×1000 for example to a size of 1000 kilobytes as an application of the bit-rate control, and image of 1000×500 as a low-frequency component of 2000×1000 in resolution can be compressed simultaneously to a size of 500 kilobytes using the feature of the wavelet transform. However, the image resulted from compression of an image as the low-frequency component of 1000×500 to 500 kilobytes is normally lower in quality than an image resulted from compression, to 500 kilobytes, of an original image whose resolution is 1000×500.